fix(installer): stop forcing setup and expose Lean bootstrap errors

[gauss-math-inc]

Summary

• stop forcing gauss setup from the generated launcher and keep installer auto mode non-interactive
• surface elan/Lean bootstrap failures with captured output and recovery hints
• extend the Ubuntu installer smoke to assert the new launcher behavior and readable bad-toolchain failures

Validation

• bash -n scripts/install-internal.sh
• bash -n tests/installer/ubuntu_repository_local_install_smoke/run-in-container.sh
• invalid toolchain repro: GAUSS_LEAN_TOOLCHAIN=leanprover/lean4:v0.0.0-opengauss-public ./scripts/install-internal.sh ... exits with a clear error, captured elan output, and a recovery command
• successful installer run against an existing Lean workspace with --skip-setup, including managed /prove staging verification and a launcher assertion that GAUSS_FORCE_FIRST_TIME_SETUP=1 gauss setup || true is absent
• interactive no-key install in auto mode completes without prompting and ends with Skipping setup wizard prompt in auto mode; run \gauss setup` later...`
• top-level ./scripts/install.sh run reaches the local template flow successfully and writes a launcher without the forced setup line

Notes

• docker is not installed in this devbox, so I did not run the Docker wrapper around the Ubuntu smoke scenario here